This invention relates to the manufacture of caustic and is an improvement in the process disclosed and claimed in parent U.S. Pat. No. 4,282,178, the full text of which is hereby incorporated herein by reference. As disclosed in said parent application, hydrazine and its derivatives and salts have been found to act as surprisingly effective corrosion inhibitors in very small concentrations when concentrated caustic solutions are made by dehydrating relatively weak solutions at a temperature between about 100.degree. and 175.degree. C. Such dehydration is commonly carried out in vessels made of nickel or nickel-containing alloys, which tend to be severely corroded by chlorate-containing caustic solutions unless a proper corrosion inhibitor is used.
While hydrazine compounds have been found to be unusually effective corrosion inhibitors in such a process, as described in said parent application, a small amount of residual hydrazine might be found in the resulting caustic product. This is generally considered harmless for most uses, but even a small residue of a hydrazine compound in the caustic solution can disqualify the latter from use in the food, cosmetic and pharmaceutical industries according to current government laws and regulations.
It has now been discovered that any such hydrazine residues can be effectively destroyed in the dehydrated caustic solution by treating the latter promptly after dehydration with a suitable amount of hydrogen peroxide or a similar suitable per-oxygen compound or with hypochlorite, as is more fully described below. Other more or less common oxidizing agents, for instance, molecular oxygen, air, permanganates, chromates or dichromates, are not practical because of the slow action of molecular oxygen or air under the conditions prevailing in the dehydration process after the evaporation steps, or because, in the case of the named salts, they impart color to the desired product or are themselves toxic, or because of a combination of such effects.
The reaction between hydrogen peroxide and hydrazine is, as such, well known in the art and has been the basis since many years ago for the use of hydrogen peroxide-hydrazine combinations as a fuel for rocket engines or high-altitude missiles. For instance, it has been reported that the Germans toward the end of World War II used as a rocket fuel an 80 percent solution of hydrogen peroxide with a mixture consisting of 30 percent hydrazine hydrate, 57 percent methyl alcohol, 13 percent water and 0.11 percent potassium cuprocyanide as a catalyst. See L. F. Audrieth and B. A. Ogg, "The Chemistry of Hydrazine," John Wiley & Sons, Inc., New York, N.Y., 1951, page 129. However, as also reported in the same reference (p.131), the reaction rate between hydrogen peroxide and hydrazine in dilute aqueous solutions was found in academic studies to be strongly affected by variations in the pH of the solutions and to rise sharply to a maximum at a pH of 10, with sharp drop-offs at both higher and lower pH values.
The present discovery of the effectiveness of hydrogen peroxide as a scavenger for traces of hydrazine in concentrated caustic solutions, i.e., in caustic solutions having a pH of at least 14 and ordinarily well above 14, is accordingly thus entirely surprising and contrary to what the prior art might lead one to expect.